Move GitHub tokens for Metrics into a separate account

[lucyb]

Currently the GitHub tokens used for the Grafana Metrics are stored in my account. It's been assigned to the relevant organisations, so other people can see it, what permissions it has (roughly) and revoke it if needed. However, the permissions list is a bit annoying and tricky to get right, so it would be better if the tokens are stored in a shared account. This would allow us to more easily recreate them and means anyone in the tech team could manage them.

Background information thread 🧵.

This process must be documented, both in the metrics repo and the tech support playbook, before this work can be considered complete.